Apparatus for direct melt syhthesis of intermetallic compounds

ABSTRACT

Method and apparatus for the melt synthesis of intermetallic compounds from metal components, at least one of which is highly volatile. The metal components are placed in a reaction vessel which is sealed by a liquid sealant in combination with a mechanical seal prior to heating the reactants to a temperature at which one or more of the reactants develops an appreciable vapor pressure. The atmosphere surrounding the reaction vessel is pressurized to a pressure above the maximum vapor pressure developed by the reactants during the melt synthesis which is accomplished by heating the reactants. The combination of mechanical and liquid seal is of a character to permit diffusion by high-pressure gas through it.

United States Patent Menashi et al.

APPARATUS FOR DIRECT MELT SYNTHESIS OF INTERMETALLIC OMPOUNDS.

Inventors: Wilson P. Menashi, Lexington; Joseph F. Wenckus, Needham;Roger A. Castonguay, Salem, all of Mass.

Arthur D. Little, Inc., Cambridge, Mass.

May 30, 1973 Assignee:

Filed:

- Applv No.: 365,135

v Related U.S. Application Data Division of Ser. No. 169,315, Aug. 5,1971, Pat. No. 3,777,009.

U.S. Cl. 266/39, 266/34 R Int. Cl. F27b 14/10 Field of Search 266/34 R,39; 75/134 R References Cited UNITED STATES PATENTS 3/1972 Deyris 23/273SP [111 3,825,242 [451 July 23,1974

Primary Examiner-Gerald A. Dost Attorney, Agent, or Firm-Bessie A.Lepper [57] ABSTRACT Method and apparatus for the melt synthesis ofintermetallic compounds from metal components, at least one of which ishighly volatile. The metal components are placed in a reaction vesselwhich is sealed by a liquid sealant in combination with a mechanicalseal prior to heating the reactants to a temperature at which one ormore of the reactants develops an appreciable vapor pressure. Theatmosphere surrounding the reaction vessel is pressurized to a pressureabove the maximum vapor pressure developed by the reactants during themelt synthesis which is accomplished by heating the reactants. Thecombination of mechanical and liquid seal is of a character to permitdiffusion by high-pressure gas through it.

16 Claims, 9 Drawing Figures PATEmEummau 3.825.242

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. r 1 APPARATUS FOR DIRECT MELT SYNTHESIS or INTERMETALLIC coMPoU spsThis is-a division of application Ser. No. 169,315,

filed Aug. 5, 1971 and now US. Pat-No. 3,777,009.

This invention relates to the synthesis of intermetallic compounds andthe like and particularly to the Group Ill-V, Group ll-Vl and GroupII-IV-VI intermetallic compounds, at least one component of which has arelatively high vapor pressure at elevated temperatures.

Although a term intermetallic is used throughout this description forconvenience, in referring to the products of the method of thisinvention, it is to be understood that this term as used hereinafter ismeant to include compounds formed of metals and noncondens- ;sure. Itisalso within the scope of this inventionto apply this'm'ethod ofimtermetallic compound synthesis to the forming of other ternary and toquaternary compounds, to mixed Group Ill-V compounds as well as to Groupll-llI-V compounds and the like.

During the past decade there has developed a growing interest in'various'uses for the Group lll-V,'Group ll-VI and Group lI IV-VIintermetallic compounds because of their unique quantum elec'tronicproperties.

- Thus these materials may be used as the detector elements in infrareddetectors,- as light-emitting diodes, as infraredwindow elements, and inGunn diodes and Hall generators. Exemplary of the Group III-Vintermetallics are those: formed by reacting one or more of such Grouplll'elements as gallium and indium with one or more of such Group'Velements as phosphorous, arsenic and antimony. Exemplary of the GroupII-VI intermetallics are those formedby reacting one or more of suchGroup. II elements as zinc, cadmiumand mercury with one or more of suchGroup VI elementsas sulfur, selenium and tellurium. Mercurycadmium-telluride, lead-tin-telluride and lead-tin-sellenide may becited'as examples of ternary systems formed of GroupIl-IV-Vl compounds.A number ofvarious dopants may also be added to these intermetalliccompounds. Because the quantum-electronic properties of any oneintermetallic.

compound depend upon the ratio of the various'ielemental componentsmaking up the-compound, it is very important to be able accurately tocontrol these ratios and to be 'able'to repr oduce them from batch tobatch."

Many of these elemental components have very-high vapor pressures atelevated temperatures both in their elemental state and in theircombined state, a fact which has made the formation of theseintermetallic compounds in their polycrystalline state exceeding difficult and costly. For example, the vapor pressure of arsenic orphosphorus over molten GaAs (m.p. 1,270 C) or GaP (m.p. 1,470C) is 1atmosphere and 35 atmospheres, respectively. The vapor pressure ofmercury over Hg C d Te can be much higher, especially 1 when the cadmiumconcentrations are relatively high with corresponding increase inmelting point.

Although the principal difficulties of producing single crystals fromthe polycrystalline materials such as the arsenides, phosphides,tellurides, selenides and the like have been largely overcome by use ofliquidencapsulated crystal growing techniques, the excessive cost of thepolycrystalline intermetallics has materially hindered single crystalmanufacture and use. Thepresent, most acceptable method for synthesizinga number of the binary intermetallic compounds in polycrystalline formfor single crystal growing is .what may be termed thehorizontal Bridgmantechnique in which the components are combined using vapor/liquid-phasereactions and then horizontally zone-refined to produce an ingot whichis subsequently cleaned and etched. For example, polycrystalline GaPproduction involves the reaction of P or PH vapor passing over moltengallium which results in a sponge-like Gal mass which generally containsexcess gallium.

The ternary mercury-cadmium-telluride compound isat present produced bya very, slow, controlled reaction of the elements in an evacuated quartzarnpoule. The reaction must be carried out very slowly with continuousrocking of the ampoule to prevent overheating and vaporizing the mercurywhich could cause the ampoule to burst. Typical reaction cycles requireabout 60 to 80 hours to form the compound. Thus, producing theintermetallic compounds by these prior art methods is time consuming andexpensive; Moreover, the quality of the ingot is difficult to controland it is generally necessary to use an excess of the volatilecomponent. Finally, the process demands the use of very small quantitiesof reactants and results in very small ingots.

I In acopending application Ser. No. 46,242 filed June -l5, 1970, inthenames of John S.- Haggerty and Joseph F. Wenckus, and assigned to thesame assignee as this application, there is described and claimed animproved method for synthesizing intermetallic compounds. By this methodthe reactants are placed in an open crucible and an encapsulant such asB 0 is pressure of the volatile component becomes suffiplaced on top ofthem; Thelcrucible is placed within a vessel which is then pressurizedto a level equal to or above the highest vapor pressure attained by thevolatile component during the reaction.'The crucible is graduallymoved'into a heating zone, e.g., within rf coils, in a manner to firstheat and melt the encapsulant and then the reactants. Through the use ofthe molten encapsulant'and the surrounding elevated pressure, itispossible 'by this method to form intermetallic compounds, inparticular those which form at the lower temperatures. However,since'the encapsulant is in physical contact .with the reactants (overand around them) it is not possible for some reactant systems to getsufficient heat into the encapsulant before the vapor it would thereforebe desirable to have available an improved method and apparatus forformin interme- 3 tallic compounds, at least one reactant of which isvolatile at elevated temperatures, wherein an encapsulant was notnecessary and by which the compounds can be formed in a relatively shortperiod of time.

It is therefore a primary object of this invention to provide animproved method for direct melt synthesis of intermetallic compoundswhich contain at least one component which is volatile at elevatedtemperatures, and particularly Group Ill-V, Group II-VI and GroupII-IV-VI intermetallics. An additional object is to provide a methodwhich permits accurate control of the composition of the compound andthe reproducibility of compositions. It is another object to provide amethod of the character described which is less expensive and lesstime-consuming than the prior art methods and which does not require anexcess of the volatile component, does not use an encapsulant and isfree from any external contamination. It is still another object toprovide such a method which lends itself to mixing during reaction andto relatively large-batch production of such intermetallic compounds;and which makes possible the incorporation of the melt synthesisdirectly into a crystal growing procedure without cooling and remeltingof the intermetallic compound.

It is another primary object of this invention to provide apparatus forcarrying out the direct melt synthesis of intermetallic compounds. Anadditional object is to provide apparatus of the character describedwhich eliminates the possibility of contamination, provides for mixingduring the formation of the compound, and makes possible relativelyrapid, large-scale batch production of intermetallic compounds ofaccurately controlled and reproducible compositions. Other objects ofthe invention will in part be obvious and will in part be apparenthereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying features of construction, combinations of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

In brief, in the method of this invention the reaction of the elementalcomponents is carried out in a reaction vessel which is sealedmechanically as well as with a molten liquid sealant. The use of themechanical seal materially reduces or eliminates the loss of vapors attemperatures below the melting point of the sealant; while the moltenliquid sealant reinforces the mechanical seal at temperatures above themelting point of the sealant. Since the molten liquid sealant remains onthe outside of the reaction vessel, the possibility of contaminating thereactants or the resulting compound is reduced. The reaction vessel isheated gradually from top to bottom while it is under sufficientpressure to minimize or eliminate any pressure differential across thewalls of the reaction vessel, thus preventing either explosion orimplosion and eliminating the need for a small heavy-walled vessel.Stirring or gentle shaking of the reaction vessel is possible for mixingof the reactants and the reaction vessel may be designed to attaindirectional solidification of the molten reaction product making itpossible in some cases to obtain directly single crystals of theintermetallic compound.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with'the accompanying drawings in which FIG. 1 is a crosssection of one embodiment of a reaction vessel of this invention showingboth mechanical and liquid seals;

FIG. 2 is a somewhat diagrammatic cross section of the working volume ofa pressure furnace with the reaction vessel in position to form themolten liquid seal;

FIG. 3 is a cross section of a second embodiment of the reaction vesselof this invention having an inert lining and illustrating the assemblyof the reactants and sealants in the vessel prior to heating;

FIG. 4 is a cross section of a third embodiment of the reaction vesselillustrating a composite vessel forming as an outer graphite susceptorportion and an inner nongraphite portion;

FIG. 5 is a cross section of a fourth embodiment of the reaction vesselillustrating a different form of mechanical seal and an contouredinterior to obtain directional cooling-of the molten reaction product;

FIG. 6 is a cross section of a fifth embodiment of the reaction vesselwherein the molten liquid sealant provides the major sealing capacity; I

FIG. 7 illustrates the incorporation of a stirrer into the reactionvessel of FIG. 6; and

FIGS. 8 and 9 show means for gently shaking the reaction vessel of FIG.1 up and down to obtain mixing of the reactants.

One embodiment of a reaction vessel suitable for carrying out the methodof this inventionis illustrated in cross section in FIG. 1. It is formedas a cylindrical enclosure 10 closed on the lower end with a bottommember 11 which has integral therewith a smaller diameter tion 17 formeasuring the temperature of the molten reactants or reaction products18 contained within the reaction chamber 19 defined within the reactionvessel.

The cylindrical enclosure 10 forming the reaction vessel is open on theupper end 20 to provide an upper access opening. The upper end hasinternal threads 21 which terminate in a shoulder 22. The reactionvessel is closed by means of a threaded plug 23 which has a centrallyextending relative thick stem 24 which may have a slot 25 for engagementwith a screw driver or the like. When plug 23 is screwed all the waydown into position, a peripheral ring of plug23 engages shoulder 22 toform, with the threaded connection, a mechanical seal for the reactionvessel. Above plug 23 and occupying at least a portion of the annularchannel 26 defined between the threaded wall and the plug stem 24 is aquantity of liquid sealant 27 which is usually a molten material such asmolten B 0 FIG. 1 illustrates the reaction vessel in position for theactual melt synthesis, i.e., positioned entirely within an rf coil 28defining a heating zone 29. FIG. 2 illustrates the reaction vesselwithin a pressurized chamber at the beginning of the melt synthesis. Theactual melt synthesis is conveniently carried out in a pressureandtemperature-controlled furnace such as that described in a copendingapplication Ser. No. 46,096 filed June 15, 1970, and assigned to thesame assignee as the present application. The furnace described in Ser.No. 46,096 permits operations to be carried out therein at temperaturesup to 4,000 C and pressures ranging from torr to 100 atmospheres. Italso provides for imparting translational and rotational motion to thelower load-bearing rod 13 as well as to an upper loadbearing rod 35 bydriving means located externally of the working volume 36 shownschematically to be defined by an enclosure 37. Pressurizing gas isintroduced through an inlet conduit 38, the fluid flow through which iscontrolled by valve 39. In a similar manner the working volume 36 is incommunication with a vacuum pump through conduit 40, the fluid flowthrough which is controlled by valve 41. The thermocouple wires 16 areconnected to a pyrometer 42, or other suitable temperature-sensing andmeasuring means; and the rf coil 28 is connected through leads 43 and 44by way of a coupling 45 to a suitable rf power supply. A temperatureandpressure-controlled furnace such as that described in Ser. No. 46,096provides all of the abovedescribed auxiliary equipment which is not partof the apparatus of this invention but which does play an important rolein describing the method of this invention.

In using the reaction vessel of FIG. 1, the weighed reactants areintroduced into the cylindrical section 10 and it is then placed, alongwith plug 23, in a glove-box. The box is evacuated and then nitorgen orsome other appropriate pressurizing gas introduced into the glovebox toa pressure of about one atmosphere. The plug 23 is then screwed into thethreaded section to make a tight mechanical seal with the shoulder 22,thus essentially retaining the pressurizing gas within the reactionvessel. The assembled vessel is removed from the glove box, a quantityof sealant (normally in solid form) is placed in the annular volume 26and the entire assembly is positioned on the load-bearing rod 13 locatedwithin the working chamber of the furnace or other suitablepressurizable device. The furnace is then pressurized with the same gasintroduced into the reaction vessel until the pressure surrounding thereaction vessel is equal to or greater than the maximum vapor pressureto be developed by the volatile component at any time during the meltsynthesis.

To begin the synthesis the reaction vessel is positioned as shown inFIG. 2 that is only that upper portion of the vessel which holds thesealant is brought within the heating zone,'e.g., within the rf coils28. The vessel is maintained in this'position until the sealant ismolten. A small amount of the sealant may work its way into the threadsbetween the plug and the vessel wall. However, even if it. doespenetrate to the'bottom leakage of the volatile component from thereaction chamber and that there can be no contamination of the reactionproduct by the liquid sealant. These facts in turn mean that thecomposition and purity of the final intermetallic compound can beaccurately controlled and reproduced.

When the sealant 27 is molten, the lower loadbearing rod 13 is graduallydriven upwardly to slowly bring the reaction vessel into the heatingzone 29 to heat and then melt the reactants. The temperature of thematerial within the reaction vessel is continually monitored by means ofthe thermocouple pyrometer and the load-bearing rod is driven at a speedso that the temperature of the reactants increases at the desired ofplug 23, its entry into chamber 19 will be blocked by the seal aroundshoulder 22. Because the sealant is positioned outside the reactionchamberl9 and is held above the level of the reactants within thechamber, it is possible to melt it and derive the benefit of its sealingability prior to the heating of the reactants to a temper-.

ature where at least one of them begins to develop an appreciable vaporpressure. This in turn means that the mechanical and liquid seals plusthe pressurized gas around the vessel are most efficiently used toprevent rate, for example at about C per hour. As the reactants withinthe vessel are heated the volatile reactant, or reactants, begins todevelop an appreciable vapor pressure. The pressures within thevvesseland around it are equalized by diffusion of gases throughthe seals sincethey are not of a quality to handle the pressures encountered. Thisequalization of pressures means that the dangers of explosions orimplosions are minimized. It also means that the reaction vessel wallsdo not have to be able to withstand large pressure differentials. Thuslittle, if any, consideration need be given to the use of a suitableratio of vessel wall thickness to reaction chamber volume; andrelatively large batches of material may be handled. As the heating iscontinued and finally when the reaction takes place a small amount ofthe volatile component, e.g., mercury, arsenic or phosphorus may be lostto the surrounding atmosphere as evidenced by a slight bubbling of theliquid sealant. However, the actual quantity issmall and may becompensated for in determining the quantities of reactants used. If thesame rate of heating is used for a series of runs to produce the sameintermetallic compound, then the composition of the final productsshould be reproducibly accurate.

Since it is only necessary that there be relative motion between thereaction vessel and heating zone, it is also within the scope of thisinvention to move theheating meansdefining the heating zone in atranslational modewhile maintaining the reaction vessel stationary orrotating it only.

The reaction within the vessel is monitored by tracing the measuredtemperature on a predetermined phase diagram chart which is used toindicate completion of the reaction. It is usually desirable to hold themelt at,

the reaction temperature for a period of time, e.g., from about to 1hour, to ensure completion of the re action. If a polycrystallineproduct is desired, then the melt product can be quenched relativelyrapidly by turning off the heating means, e.g., the power to rf coils.In some casesit may be desirable to repeat the heating and cooling cycleto ensure complete reaction. The product is removed as an ingot from thereaction vessel. Alternatively, the melt may be directionally solidifiedusing the Bridgman-Stockbarger method to produce a single crystal or alarge polycrystal ingot. A.

reaction vessel suitable for this is illustrated in FIG. 5.

The reaction vessel may be constructed from a number of differentmaterials including graphite, boron nithe reaction vessel can beconstructed of graphite to serve as a susceptor for the rf radiation.However, if the reactants, or the reaction product, are of a nature toreact with or be contaminated by graphite, then the vessel and plug maybe lined with an inert material as shown in FIG. 3. Heating means otherthan an rf coil may be used. Such heating means include, but are notlimited to, resistance heaters, lasers, and the like.

In FIG. 3, wherein like reference numerals are used to refer to likecomponents, the inner walls defining the reaction chamber 19 are linedwith a liner 50 formed of a suitable inert material; and the end 51 ofplug 23 is also lined or suitably coated. Although sleeve-type linersare illustrated in FIG. 3, the liners may be appropriately thickcoatings applied by painting, spraying and the like. FIG. 3 alsoillustrates the assembling of the reaction vessel, showing the solidreactants 52 within the vessel and the solid sealant material 53 placedin position on and around the plug.

The reaction vessel of FIG. 4 is a modification of that of FIG. 3. Inthe vessel of FIG. 4, the reaction vessel is formed of a material otherthan graphite and it is set in a graphite sheath 55. FIG. illustratestwo further modifications where the plug is not threaded and wherein thereaction chamber is shaped to achieve directional cooling. The plug 56is cut without threads and a threaded ring 57 is used to force the plugto contact shoulder 22. The liquid sealant in such a case may penetrateinto the annular passage 60 defined between the inner wall of ring 57and the plug stem 24. The main cylindrical enclosure 58 of the reactionvessel is constructed to define a conically-shaped bottom 59 inthereaction chamber. The molten reaction product may be cooled by graduallywithdrawing the reaction vessel from the heating zone so that coolingand solidification begins at the bottom tip of the cone. This achievesdirectional solidification by a form of the Bridgman Stockbarger method.

The embodiments of the reaction vessel illustrated in FIGS. 6 and 7provide for a somewhat different arrangement to achieve mechanical andliquid seals. The main body of the vessel is formed of a cylindricalsection 65 which is integral with an upper thick-walled section 66. Thissection 66 has an annular sealant channel 67 cut into it. This channelhas an upper section 68 with a frustoconical cross section and a lowersection 69 with a rectangular cross section. The reaction vessel issealed with a lid 70 which is formed of a ring section 71, the wall ofwhich has a thickness corresponding to the width of channel section 69,and a cover member 72. The cover member 72 has a centrally positionedattachment collar member 73 through which the lid is attached to anupper load-bearing rod 35 provided in the pressurizable furnace. Upperrod 35 may be driven to experience both translational and rotationalmotions. In using the reaction vessel of FIG. 6, the solid particles ofsealant material 53 are placed into the sealing channel 67, the vesselis mounted on the lower rod 12 and the lid affixed to upper rod 35. Thereaction vessel can not be evacuated and then filled with a pressurizinggas in a glove-box such as described for the use of the vessel of FIG. 1(or for FIGS. 3-5). However, evacuation and introduction of anatmosphere or so of pressurizing gas can readily be accomplished withinthe furnace volume (see FIG. 2). The lid may then be set to just rest onthe sealant particles and heating begun so that the sealant melts. Atthis time upper rod 35 is moved downwardly to seat the lid firmly ontothe reaction vessel. As

the reaction vessel is moved upwardly into the heating zone, both of therods 13 and 35 are moved at the same rate to maintain the firm contact.In the embodiment of FIGS. 6 and 7, the liquid seal provides the majorpart of the sealing. The sealant channel 67 should be of a sufficientdepth to hold enough sealant so that it may provide the required sealingability without reaching the top of the channel to spill into chamber19.

FIG. 7 illustrates the reaction vessel of FIG. 6 equipped with a stirrerwhich is affixed to the lid 72. Relative motion between the lid and theremaining part of the reaction vessel is then attained by rotating theupper rod 35 and the lid therewith. It is also, of course, possible torotate the lower rod 13 rather than upper rod 35.

The reaction vessel of FIGS. 8 and 9 provides an alternative form ofmixing, namely a gentle up and down motion. The vessel is essentiallythe same as is shown in FIG. 1, except that the lower support section iscontoured on the bottom to provide a cam surface 86 and the lowerload-bearing rod 13 has a cam follower in the form of a horizontal bar87. The plug stem 88 has two opposed slots 89 and 90 in which a pin 91,affixed to upper rod 35, moves to maintain alignment of the reactionvessel and prevent its rotation as it is moved up and down with therotation of lower rod 13 and the engagement of cam follower rod 86 withcam surface 86. Mixing by whatever means employed may be continuous orperiodic.

It is, of course, within the scope of this invention to combine featuresof the reaction vessels illustrated, for example to use the sealingmeans of FIG. 1 with the vessel of FIG. 5; to use the sealingmeans ofFIG. 5 with the mixing means of FIGS. 8 and 9, etc.

A number of different sealants may be used. These include, but are notlimited to boric oxide (B 0 barium oxide and these oxides in admixturewith barium chloride and sodium fluoride, potassium chloride, sodiumchloride and the like. The density of the molten sealant is notimportant, nor is its reactivity with the metal reactants or resultingreaction product important. The sealant should, however, be a compoundwhich melts below the reaction temperature and which has very low vaporpressure over the temperature range at which it is used.

The following example, which is meant to be illustrative and notlimiting, further describes the method of this invention. A mixture of103 grams of mercury, 18 grams of cadmium and I02 grams of tellurium wasloaded into a graphite crucible constructed as shown in FIG. 1. Thepreliminary mechanical sealing was accomplished in a glove-box after thebox was evacuated and filled to about 1 atmosphere of nitrogen. Thesealed reaction vessel was then transferred to a pressureandtemperature-controlled furnace constructed as described in Ser. No.46,096. The furnace was pressurized with nitrogen to 1150 psig andheating was begun by first melting the B 0 used as a liquid sealant.After the B 0 was melted, heating was continued, by moving the vesselupwardly through an rf coil, at a rate of about C/hour until thetemperature within the vessel was raised to something over 810 C, thecalculated reaction temperature. The charge was held at that temperaturefor about 20 minutes and then cooled rapidly by shutting off the powerto the rf coil. A polycrystalline ingot 1% inches in diameter and 1%inches long was obtained. The original mixture of reactants was formed I9 to produce Hg Cd Te, taking into account the need for excess mercury,the volatile component.

-In addition to providing reactants in solid form, the pressurizing gasmay itself be a reactant, e.g., nitrogen to form nitrides. Any suitabledopants may be added to the reactants.

By sealing the reaction vessel, both mechanically and with a liquidsealant, in a manner to permit the diffusion of high-pressure gasesthrough the seals it is possi ble to maintain the pressures within andoutside of the reaction vessel at essentially the same level, thuseliminating the need for a thick-walled vessel and thus permittingrelatively large batches to be handled. By using a liquid sealantinstead of an encapsulant and by maintaining the sealant above thereactants and out of contact with them is is possible to make full useof the scaling properties of the sealant before any appreciable amountof vapor pressure is developed by one or more of the reactants and toprevent any contamination of the reaction product.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attainedand,since certain changes may be made in carrying out the above method andin the constructions set forth without departing from the scope of theinvention, it is intended thatall matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

We claim:

l. A reaction vessel suitable for carrying out melt synthesis,comprising in combination lar channel cut in the rim of said enclosuremeans around said access opening, said annular channel having a lowerrectangular cross sectional portion and an upper frustoconical crosssectional portion; and wherein said closure means comprises a lid havinga lower annular sealing ring of a thickness adapted to form'a close fitwith said lower portion of said channel.

9. A reaction vessel in accordance with claim 8 including stirring meansaffixed to said lid and extending into said reaction chamber wherebymotion of said lid a. enclosure means defining a-reaction chamber andhaving an upper access opening; b. closure means engagable with saidupper access opening thereby .to form a'mechanical seal; and

0. liquid sealant channel means associated with said mechanical seal todefine therewith a combination liquid-mechanical seal of a characterwhich prevents said liquid sealant from entering said reaction chamberbut which permits the diffusing of highpres'sure gases therethrough.

2. A reaction vessel in accordance with claim 1 wherein said upperaccess opening and said closure means are threadably engagable.

3. A reaction vessel in accordance with claim 1 wherein said liquidsealant'channel means is located above said mechanical seal.

4. A reaction vessel in accordancewith claim 1 wherein said liquidsealant channel means is located below said mechanical'seal. 5. Areaction vessel in accordance with claim 1 wherein said upper accessopening'has internal threads I and terminates at its lower end inannular shoulder means. U 1

6. A reaction vessel in accordance with claim 5 wherein saidclosuremeans comprises a threaded plug engagable with said shoulder means andhaving a central stem which defines with the walls of said access- 1. aplug engagable with said shouldermeans and having a central stem, and

relative to said enclosure means will effect the mixing of the contentsof said reaction-chamber.

10. A reaction vessel in accordance with claim 1 further characterizedby having supporting means associated with the bottom of said enclosuremeans and being adapted for engagement with load-bearing rod means.

11. A reaction vessel in accordance with claim 1 further characterizedby having thermocouple channel means terminating'within the wall of saidenclosure means and adapted to measure the temperature obtaining withinsaid reaction chamber.

12. A reaction vessel in accordance with claim l further characterizedby having coupling means associated with said closure means adapted tocouple said closure means with an upper load bearing rod.

, 113. A reaction vessel in accordance with claim 1 furthercharacterized by having 1. supporting means associated with the bottomof said enclosure means, the lower surface of said supporting meansbeing contoured to form a cam surface engagable with cam follower meansaffixed to a lower load-bearing rod capable of being driven in bothrotational and translational modes, whereby rotation of saidload-bearing rod imparts an upward-downward motion to said reactionvessel; and

2. opposing slot members in the upper end of said closure means adaptedto engage a horizontal rod member affixed to an upper load bearing rodwhereby said reaction vessel is maintained in alignment and preventedfrom rotation during said up-' ward-downward motion.

14. A reaction vessel in accordance with claim 1 wherein said reactionchamber is configured to terminate at its lower end in a conical.section the apex of which forms the lowest point of said chamber;

15. A reaction vessel in accordance with claim 1 wherein said enclosuremeans is constructed at least in part of graphite.

' 16'. A reaction vessel in accordance with claim 1 wherein saidenclosure means includes inert linear

1. A reaction vessel suitable for carrying out melt synthesis,comprising in combination a. enclosure means defining a reaction chamberand having an upper access opening; b. closure means engagable with saidupper access opening thereby to form a mechanical seal; and c. liquidsealant channel means associated with said mechanical seal to definetherewith a combination liquid-mechanical seal of a character whichprevents said liquid sealant from entering said reaction chamber butwhich permits the diffusing of highpressure gases therethrough.
 2. Areaction vessel in accordance with claim 1 wherein said upper accessopening and said closure means are threadably engagable.
 2. a threadedring adapted to force said plug into engagement with said shouldermeans; and wherein said liquid sealant channel is defined above saidthread ring between said stem and the internal wall of said accessopening.
 2. opposing slot members in the upper end of said closure meansadapted to engage a horizontal rod member affixed to an upper loadbearing rod whereby said reaction vessel is maintained in alignment andprevented from rotation during said upward-downward motion.
 3. Areaction vessel in accordance with claim 1 wherein said liquid sealantchannel means is located above Said mechanical seal.
 4. A reactionvessel in accordance with claim 1 wherein said liquid sealant channelmeans is located below said mechanical seal.
 5. A reaction vessel inaccordance with claim 1 wherein said upper access opening has internalthreads and terminates at its lower end in annular shoulder means.
 6. Areaction vessel in accordance with claim 5 wherein said closure meanscomprises a threaded plug engagable with said shoulder means and havinga central stem which defines with the walls of said access opening saidliquid sealant channel.
 7. A reaction vessel in accordance with claim 5wherein said closure means comprises
 8. A reaction vessel in accordancewith claim 1 wherein said liquid sealant channel comprises an annularchannel cut in the rim of said enclosure means around said accessopening, said annular channel having a lower rectangular cross sectionalportion and an upper frustoconical cross sectional portion; and whereinsaid closure means comprises a lid having a lower annular sealing ringof a thickness adapted to form a close fit with said lower portion ofsaid channel.
 9. A reaction vessel in accordance with claim 8 includingstirring means affixed to said lid and extending into said reactionchamber whereby motion of said lid relative to said enclosure means willeffect the mixing of the contents of said reaction chamber.
 10. Areaction vessel in accordance with claim 1 further characterized byhaving supporting means associated with the bottom of said enclosuremeans and being adapted for engagement with load-bearing rod means. 11.A reaction vessel in accordance with claim 1 further characterized byhaving thermocouple channel means terminating within the wall of saidenclosure means and adapted to measure the temperature obtaining withinsaid reaction chamber.
 12. A reaction vessel in accordance with claim 1further characterized by having coupling means associated with saidclosure means adapted to couple said closure means with an upper loadbearing rod.
 13. A reaction vessel in accordance with claim 1 furthercharacterized by having
 14. A reaction vessel in accordance with claim 1wherein said reaction chamber is configured to terminate at its lowerend in a conical section the apex of which forms the lowest point ofsaid chamber.
 15. A reaction vessel in accordance with claim 1 whereinsaid enclosure means is constructed at least in part of graphite.
 16. Areaction vessel in accordance with claim 1 wherein said enclosure meansincludes inert linear means, the walls of which define said reactionchamber.